A low-dispersion glass is used for lenses that constitute an optical system for correcting the chromatic aberration, and the like. A typical low-dispersion glass is a fluorophosphate glass as described in JP-A-6-191876.
A lens formed of a fluorophosphate glass has a high utility value as described above, while a more space-saving optical system can be realized when the optical-function surface of the lens is aspherical. The precision press molding method is suitable for producing such an aspherical lens.
The precision press molding method enables the mass-production of not only aspherical lenses but also lenses that are difficult to produce by a polishing method in which the optical-function surface is completed by polishing.
Meanwhile, when a lens is produced by the precision press molding method, there is employed a method in which the external form and optical-function surface of the lens are formed by the precision press molding and the circumferential portion of the optical-function surface is cut or ground to complete the lens. The above cutting or grinding of the circumferential portion is called a centering and edging process, in which the lens is completed to ensure that a portion called a round edge is in parallel with the optical axis.
When the centering and edging process is carried out, the round edge is cut or ground while the optical-function surface is held with a fixing tool. In this case, however, a precision-press moldable fluorophosphate glass has a problem that the optical surface thereof is easily scratched during the above centering and edging process since the hardness of the glass is low.